Physical vapor deposition (PVD) processes involve the formation of a coating on a substrate by physical deposition of atoms, ions, or molecules of the coating species. Traditionally, three main techniques for applying PVD coatings have been primarily used by industry: thermal evaporation, sputtering, and ion implanting. Thermal evaporation involves heating of the material until it forms a vapor that condenses on a substrate to form a coating. Sputtering involves the electrical generation of plasma between the coating species and the substrate. Ion implantation is essentially a combination of these two processes. Common features of all three techniques include the need for high vacuum and high power requirements.
Traditional PVD processes produce coatings for a range of applications including electronics, optics, decoration, and corrosion and wear resistance. These PVD coatings can be pure metals, alloys (via co-deposition), or compounds (via co-deposition or reactive deposition in a gaseous environment). Traditional PVD coatings are characteristically thin (up to a few μm), and are deposited at relatively low rates (1-10 nm/s). Most processes are operated on a batch basis, and the component size is limited by the size of the vacuum chamber. Provided that the substrate can be manipulated to face the coating source, the size and shape of objects to be coated are limited by the capital and operating expenditures involved rather than by the fundamental characteristics of the process.
In thermal evaporation, vapors are produced from a material located in a source that is heated by various methods such as resistance, induction, arc, electron beam, or laser. The substrate is located at an appropriate distance facing the evaporation source and the gas atoms condense on the relatively cold substrate surface. Thermal evaporation is typically conducted in vacuums of between 10−9 to 10−5 torr and under conditions where an atom evaporating from the source material travels in a straight line. Therefore, the process is line-of-sight limited, and coating around corners or angles is not possible without substrate manipulation.
Due to these drawbacks, traditional processes are generally unsuitable for providing coatings on the interior of hollow objects, such as pipes, in a cost effective and simplified manner. At the same time, a variety of industrial products stand to benefit from cost effective coating techniques that could apply coatings to the interior of hollow objects.
For example, new EPA regulations were placed into effect in October of 2002 governing diesel engine emissions. These regulations compel engine manufacturers to install exhaust gas recirculators (EGRs) on all their heavy-duty diesel engines. To minimize the weight of these systems, it is desirable to fabricate EGRs primarily from Al alloys. Unfortunately, sulfuric and nitric acid present in the exhaust gas can cause significant corrosion within aluminum tubing. Fe-rich iron aluminides coating on the interior these tubes could significantly reduce the corrosion due to their superb oxidation and sulfidation resistance.
Another example is the need for alternatives to Cr electroplating for wear- and corrosion-resistant coatings on the inner surface of medium-caliber (5.56 mm to 45 mm) gun barrels, due to environmental and health issues associated with hexavalent Cr electroplating. Another example is related to furnace tubes used in ethylene production. During ethylene production, coke deposits are formed on the inner surfaces of the furnace tubes, reducing heat transfer and degrading production. The deposits are typically removed by heating in excess of 1000° C., which causes downtime for production as well as significant energy costs. In addition, high-temperature alloys are required for the tubes to avoid damage during the coke removal process. Presently, DOE-OIT is funding a handful of studies under the Industrial Materials of the Future program to evaluate a variety of solutions to the coking problem, including coking-resistant alloys, liners, coatings, and novel tube geometries. However, none of these approaches have yet provided a cost-effective solution to the problem. Recent work by Oak Ridge National Laboratory and others suggest that a Fe-rich iron aluminide coating on a less-expensive furnace tube alloy would offer significant improvement in coking resistance, thereby reducing or eliminating the need for downtime and high-temperature de-coking treatments. However, since the ethylene furnace tubes are often required to be up to 30 ft long, improved methods of providing Fe-rich iron aluminide coatings on the interior surfaces of these tubes are still required. Yet another example relates to the development of advanced coal-fired power generation systems such as pressurized fluid-bed combustion (PFBC) systems and integrated gasification combined cycles (IGCC) systems. These technologies can provide economical power generation with minimal environmental emissions and high efficiency. However, these advanced power generation projects require a hot gas filter to remove corrosive and erosive coal ash entrained in the combustion stream. These hot gas filters, or candle filters, must be cost-effective and able to withstand the effects of corrosion, elevated temperature, thermal shock, and temperature transients. Degradation of metallic filter elements (typically austenitic stainless steels) has been observed under oxidizing, sulfidizing, and/or carburizing conditions, and acts as a driving force for the development of alternate hot gas filter materials. Iron aluminides can be considered for such applications because of their excellent high-temperature corrosion resistance in a variety of sulfur-bearing environments relevant to coal-derived systems. Other alternatives include ceramic or advanced alloy filters. However, use of these materials entails other considerations such as difficulties in fabrication, strength, and thermal shock resistance. Many of these problems could be overcome by coating the interior surfaces of inexpensive metallic filters with an appropriate corrosion-resistant material such as Fe-rich iron aluminides or Ni-Cr-Al-Fe alloys.
These and other applications create a need for cost effective techniques and technologies that enable the formation of coatings on the interior surfaces of hollow objects. More generally, there exists a need for methods that allow the formation of tightly bonded diffusion coatings on the interior surfaces of hollow metallic objects, such as pipes and tubes.